Method for preparation of absorbing substances

ABSTRACT

The invention relates to a method for preparing absorptive substances from lignocellulosic materials, such as straw of cereal plants, peels or hulls of cereal grains, plant leaves, bagasse, jute or wood chips. The method according to the invention comprises as its essential stages an alkaline pretreatment for partial removal of lignin and hemicellulose, an initiation treatment for forming reactive radicals, addition of at least one monomer and cross-linking agent, and finally a polymerization. As a preceding treatment, washing with water for removal of extraneous matter and/or disturbing components can be included, and potentially a wet milling or another defibration for increasing the reactive surface. For initiation, an oxidative chemical such as hydrogen peroxide is suitable, for the monomer especially compounds containing a vinyl group, such as acrylic acid. For obtaining or improving ion exchange properties, an ionizable atomic group can be included either as a part of a monomer, or forming it after the polymerization.

[0001] The objective of this invention is a method for preparing absorptive substances from lignocellulosic materials such as straw, peels or hulls of cereal crop, plant leaves, wood chops, bagasse or jute.

[0002] The most important applications of absorbents are absorbing and release of water, flocculation of colloids, filtering aids, removal and a possible recovery of harmful organic or inorganic compounds, based among others on the ion exchange properties of absorbents, and controlled release of medicines and agrochemicals. The range of applications is wide from hygiene and hospital articles to various purposes in the industry, agriculture and environmental protection.

[0003] In addition to the traditional absorbers such as Fuller's earth, other silica minerals, silica gel and activated carbon, synthetic or semi-synthetic organic absorbents or so-called super absorbents have been developed during the past few decennia. At the first stage of this development, the starting material was starch, to which hydrophilic and water-absorbing atomic groups were added by using grafting techniques. These groups can be positively or negatively charged. The most usual grafting chemicals were acrylic acid, methacrylic acid and their derivatives such as salts, esters, amides and nitrites. The quality property most often followed has been the water absorbing capacity. It was determined initially using excess of water under atmospheric pressure, and separating the solid matter from the mixture by centrifugation, later by following the absorption under pressure and in salt solutions, thus imitating properties which are essential when used for hygiene or hospital articles.

[0004] The starch-based products developed initially could have water absorbing capacity of several hundred times that of the weight of the dry absorbent. However, alone with the increase of the amount water absorbed, mechanical properties of the gel formed were weakened, and a substantial part of the water was released under pressure. Since hygiene articles have been a major field of application of the organic absorbents, the main part of the demand has been directed to absorbents, which can imbibe and hold water and dilute solutions such as blood and excreta even under mild pressures. A similar quality requirement is valid also for substances used for absorbing and release of water in agricultural and horticultural applications. This has directed the development and marketing towards fully synthetic absorbents, where the quality required is often presented to be a 25 to 35-fold absorption of a physiological saline solution under a pressure of 0.2 bars. Only few published data and applications exist on absorbing other materials but water in these absorbents.

[0005] For maintaining the absorbing capacity under pressure, cross linkages have to be created in the polymer. They diminish the total absorbing capacity. Cross linkages also form steric hindrances for continuation of the water absorption, and limit the penetration of water deeper than in the surface layers. Also in the cross-linked materials, a high water content causes a weakening of the mechanical properties of the gel. Diminishing the particle size causes easily agglomeration problems and makes it difficult to maintain an even distribution of the absorbing material to the other components of the final absorbing product. For these reasons, also fibrous or foil-formed absorbers have been prepared. Fibres can be formed by polymerizing the same monomer or from another synthetic polymer. Alternatively, an absorbing layer can be prepared on the surface of isolated natural polymers such as cellulose or wool fibres. A fibrous absorbing material can be bound to other fibrous materials by weaving or by using non-woven techniques known as such. Advantages of using fibrous materials are an easy separation from the liquid phase, which enables uses similar to filtering materials, or when mixed in large amounts of liquids, a separation merely by sedimentation.

[0006] A hydrogel forming polymer, 2-methyl hydroxyethyl methacrylate (HENIA) has been grafted on the surface of cellulose fibres, polyethylene, or silicon rubber. When using cellulose fibres, initiators used for grafting have been light, γ-irradiation, or chemical initiation. Weaknesses of these methods are, according to Karlsson and Gatenholm (Polymer 38, 4727-4731, 1997), high equipment costs, a weak controllability of the polymerization, and difficulties to avoid homopolymerization. These researchers have used for chemical initiation a treatment with ozone on the surface of moistened cellulose fibres, this treatment forming hydroperoxides on the surface. Polymerization is subsequently formed in methanol solution under nitrogen atmosphere. The duration of the ozone treatment has been 90 minutes. This treatment, however, caused breaking of cellulose molecules thus weakening the mechanical properties of the fibrous material. Grafting started from the pores and crevices of the surface, and for a complete covering of the surface with the hydrophilic layer, an amount of grafting material higher than 100% of the weight of the cellulose treated was needed. Absorbing properties of the material obtained have not been published.

[0007] Graft polymerization has also been applied on non-isolated natural fibre materials. Mohanty, Parija and Misra (Journal of Applied Polymer Science 60, 931-937, 1996) graft polymerized acrylonitrile on the surface of pineapple leaves, which had been previously treated chemically. The pretreatment consisted of three extraction stages, followed by treatments with ethylene diamine and zinc chloride. Washing and drying operations were performed between all these stages. As initiators for polymerization, cerium (IV) sulfate and N-acetylglycine were used, and the reaction was performed under nitrogen atmosphere. The amount of grafted material varied depending on experimental conditions from 59.8 to 114.3% of the weight of the fibrous material. Grafting was reported to increase the thermal stability of the material, but data on other properties have not been published.

[0008] As by-products of cereal industries and agriculture, large amounts of lignocellulosic material such as straw, peels and hulls are formed. They have been used as absorbents as such or after some simple chemical treatments. The water absorbing capacity of untreated material is weak, being maximally two parts by weight per one part of the dry absorbing material. For this reason, this material as such is economically feasible for absorbing purposes only for absorbing excreta of cattle or poultry. Its water absorbing capacity can be elevated by treatments with alkali or by a combined treatment with alkali and peroxides, as it is presented in the U.S. Pat. No. 4,806,475. Fibre preparations obtained by such treatments are marketed as water absorbing food additives. Their water absorbing capacity is 6 to 8 parts per one part of dry matter of the absorbing material. No data exist on their water absorbing capacity under pressure, and these materials are not marketed, for example, for preparation of hygiene articles.

[0009] In the present invention it has been surprisingly found, that such easily and economically available lignocellulosic materials can be in a relatively simple process converted to absorbers having a high absorbing capacity especially under pressure. Characteristic for the method developed for preparing such absorbers is that it comprises the following steps:

[0010] (a) lignocellulosic material is treated with alkali to remove a part of its lignin and/or hemicellulose content,

[0011] (b) after step (a), the material is treated to provide its cellulose content with reactive radicals capable of functioning as polymerization initiators,

[0012] (c) after step (b), at least one polymerizable monomer and at least one cross-linking agent are added to the material,

[0013] (b) the composition obtained at step (c) is polymerized.

[0014] According to the invention, preparation of an absorbent is performed advantageously by initiating the treatments by water washing of the lignocellulosic material, whereby finely dispersed and water-soluble compounds are removed, and among others, the contents of starch and protein are reduced. The pretreated material is now treated with an alkaline solution and a treatment with hydrogen peroxide, persulfate, or another strongly oxidizing treatment for enabling the fixation of the polymer and for initiating the polymerization. Into a moist material, one or several monomers and cross-linking agents, separately or previously mixed, are added, and the polymerization is performed at a temperature below 75° C.

[0015] Straw, peels, hulls or another lignocellulosic material deriving from an industrial process can contain extraneous material such as soil, and starch and proteins deriving from other materials such as cereal grains. Since these materials can weaken the fixation of the polymer formed and/or inactivate radicals formed for initiating the reaction, it is advantageous to preclean the material for removal of extraneous compounds. A great part of the said impurities can be removed by washing with water. When materials with a waxy surface are used, the wax has to be removed. This can be most efficiently performed by a solvent treatment.

[0016] The purpose of the alkali treatment is to remove from the lignocellulosic material hemicellulose, lignin and other phenolic compounds, which could at the following stage hamper or disturb the initiation by capturing radicals, and by weakening the fixing of the polymer formed onto the fibres. A substantial cost advantage is however obtained thereby, that according to the invention these materials or components are only partially removed, without an attempt to purify the cellulose completely from other components. By alkali treatment and the possibly preceding water washing 30 to 95%, advantageously 50 to 80% of the total amount of lignin and hemicellulose can be removed.

[0017] Preferential starting materials such as cereal straw, peels and hulls, are fibrous or foil-formed in the structure. Additionally, it can be advantageous to separate the fibres in order to increase the reactive surface in relation to the total weight, and to modify physical properties of the material according to the requirements of the end use. In case a chemical defibration would be performed, the costs would be easily high and the yield would remain at about 40% of the initial weight. The material obtained would not be competitive as compared to cellulose from the present large-scale industrial processes. An analogous defibrating effect can however be obtained by treating the material according to the invention with alkali, whereby the main part of hemicellulose and a substantial part of lignin is dissolved and removed. An effective defibrating is achieved especially by treating with a strong alkaline solution at a temperature under 40° C. Economical defibrating treatments are also mechanical wet-millings in water suspensions, and chemo-mechanical wet milling, both of which can be performed at temperatures from 0 to 100° C., or at higher temperatures under pressure using, for example, extrusion techniques. The advantage of chemo-mechanical wet milling as compared to alkaline extraction without milling is a lower consumption of chemicals and a more effective defibrating, the disadvantage is some disruption of the fibres in the process.

[0018] The initiation treatment whereby reactive radicals are formed is in this invention performed advantageously by using an oxidative chemical, such as hydrogen peroxide or sodium persulfate. This stage is followed by addition of one or several monomers and cross-linking agents to the moist material, preferentially without any washing or other intermediate stages. Polymerization can be accelerated by heating the reaction mixture, maintaining the temperature, however, below 75° C.

[0019] Monomers to be used in this invention can be one or several compounds containing a vinyl group, such as acrylic acid, methacrylic acid, styrene, N-vinyl pyrrolidone, or their derivatives. Choice of the monomers and cross-linking agents depends on the properties desired for the end product, such as ion exchange properties, water binding capacity and the effect of acidity, ionic strength, and pressure on these properties.

[0020] The properties of the product can also be influenced by down stream treatments following the polymerization. Thus, for example, when acrylic acid is used as a monomer, weakly dissociating carboxyl groups which can act as ion exchangers are formed in the polymer layer, and the water absorbing capacity can be elevated by treatments with alkali, whereby a part of these groups are neutralized. Strongly dissociating cation exchanging atomic groups can be obtained by using as one of the monomers vinyl monomers which contain a strongly or intermediately strongly dissociating atomic group such as sulfonic acid group. Alternatively, the product after the polymerization can be subjected to a treatment whereby such groups are formed, for example by treating with chlorosulfonic acid. Correspondingly, anion exchange properties can be obtained in the product by using as the monomer or as one of the monomers a vinyl compound containing basic atomic groups, or by performing after the polymerization a treatment whereby such groups are formed or introduced, according to methods known as such.

[0021] The experimental material used in the investigations on which this invention is based has been oat hulls. Its content of cell walls is as a mean more than 83%, its content of lignin being below 10%, of cellulose 30 to 35%, and of hermicellulose 30 to 35%, respectively (Welch, Journal of the Science of Food and Agriculture 34, 417-426, 1983).

[0022] Implementation of the method is described in the following examples.

EXAMPLE 1

[0023] 100 parts by weight of oat hulls obtained from an industrial dehulling process were extracted for 2.5 hours in water heated to the boiling point. The water phase containing also the finely dispersed fraction was separated. The separated fraction contained 10.25 parts by weight of dry matter. The wet solid fraction was extracted during three hours in a solution containing one part by volume of ethanol and two parts by volume of toluene, at the boiling point of the mixture. The solids were separated. The drying residue of the solution was 1.45 parts by weight, and it consisted mainly of lignin and of a small proportion of carbohydrates. The latter result indicates that the amount of waxy compounds in this material is negligible, and thus their removal by extraction is not needed.

EXAMPLE 2

[0024] To two parts by weight of the extracted and dried material from Example 1, 50 parts by weight of 23% potassium hydroxide were added, and the mixture was kept at room temperature (23° C.) for 18 hours. The solution was separated by decanting. Dry matter of the solids was 58% of the weight of the extracted and dried material taken for treatment in this example. Its water absorption capacity was determined by immersing in distilled water and by removing the non-absorbed water by centrifugation (2000×g for 0.5 hours). The water absorption capacity was sixfold as compared to the dry weight.

EXAMPLE 3

[0025] The mixture of oat hulls and potassium hydroxide obtained in Example 2 was diluted to a twofold volume with distilled water, and one part by weight of 30% hydrogen peroxide was added. Mixing was continued at room temperature for three hours. The solution was removed by decanting. After this treatment, the dry weight of the solids was 84.3% of that taken for the treatment in this example. Its water absorbing capacity, determined as in the example 2, was 7-fold as compared to the dry weight.

EXAMPLE 4

[0026] The solids after decanting in the Example 3 were transferred without any preceding washing into a reaction vessel. 2.38 parts by weight of redistilled acrylic acid and 0.13 parts by weight of redistilled ethyleneglycol dimethacrylate (EDMA) were added. Air was removed by leading argon gas through the reaction mixture for 5 minutes, and 0.04% by weight of sodium persulfate were added. The temperature was elevated to 60° C., and polymerization was continued for 1.5 hours, maintaining the temperature of the mixture below 75° C. The polymer formed was cooled, washed with a 0.0125 mol/L sodium hydroxide solution, separated from the solution by filtering under vacuum, and dried in vacuum. The water binding capacity, as measured with a 0.9% sodium chloride solution under pressure, was 16.5 fold as compared to the dry weight.

EXAMPLE 5

[0027] The experiment according to Example 4 was repeated by using fibrous material obtained from a treatment according to Example 2 as starting material. The product obtained had 12.5 fold water binding capacity as compared to the dry matter, when tested under pressure.

EXAMPLE 6

[0028] The experimental serie according to Examples 2 to 4 was repeated in a modification where under the alkali treatment the temperature was elevated to 40° C. for one hour, after which the solution was removed by decanting, and the duration of the hydrogen peroxide treatment was one hour. The product obtained had a 18-fold water binding capacity as compared to the dry matter, when tested under pressure.

[0029] The examples indicate the operation principles, but do not limit ingredients or their proportions in the implementation. They may be selected depending on the physical form and functional properties desired.

[0030] In addition to straw, peels and hulls of cereal crops, the method can be used to treat other lignocellulosic materials, which either are in thin layers or can be brought to thin layers. Examples of other materials are wood chips, bagasse, jute and leaves of plants.

[0031] The dissolved material obtained at the stages described in Examples 1 and 2 is a by-product which can be recovered and marketed separately, based on its high content of hemicellulose, for industrial raw materials or for feeds. 

1. A method for the preparation of an absorbing substance from lignocellulosic material, characterized by steps in which: (a) lignocellulosic material is treated with alkali to remove a part of its lignin and/or hemicellulose content, (b) after step (a), the material is treated to provide its cellulose content with reactive radicals capable of functioning as polymerization initiators, (c) after step (b), at least one polymerizable monomer and at least one cross-linking agent are added to the maerial, (d) the composition obtained at step (c) is polymerized.
 2. A method according to claim 1, wherein the lignocellulosic starting material is fibrous or foil-formed.
 3. A method according to claim 2, wherein the lignocellolosic material is straw, peels or hulls of a cereal crop.
 4. A method according to claim 1, wherein before the alkali treatment of step (a), the lignocellulosic material is washed with water for removal of impurities.
 5. A method according to claim 1, wherein by the alkali treatment of step (a) and a possible preceding water washing, 30 to 95%, preferably 50 to 80% of the sum of lignin and hermicellulose present in the starting material is removed.
 6. A method according to claim 1, wherein at step (a) or before it, lignocellulosic material is defibrated by subjecting it to wet milling.
 7. A method according to claim 1, wherein the lignocellulosic material is defibrated by using an extrusion operation.
 8. A method according to claim 1, wherein the initiation treatment of step (b) for forming reactive radicals is performed by treating with an oxidative chemical.
 9. A method according to claim 8, wherein the initiation treatment is performed by treating with hydrogen peroxide.
 10. A method according to claim 8, wherein the initiation treatment is performed by treating with sodium persulfate.
 11. A method according to claim 8, wherein after the alkali and oxidation treatments at steps (a) and (b), the moist solid material is subjected to polymerization without any intermediate washing treatment.
 12. A method according to claim 8, wherein one or several monomers and cross-linking agents are added at step (c) simultaneously.
 13. A method according to claim 12, wherein one or several monomers and cross-linking agents are added as a mixture.
 14. A method according to claim 12, wherein the monomer or monomers are compounds, which contain a vinyl group.
 15. A method according to claim 14, wherein the monomer is acrylic acid, methacrylic acid or their derivative.
 16. A method according to claim 1, wherein the monomer contains an ionizable atomic group, which does not participate in the polymerization reaction.
 17. A method according to claim 1, wherein the polymer obtained from step (d) is subjected to a reaction for forming an ionizable atomic group in it.
 18. A method according to claim 16 wherein the ionizable atomic group is sulfonic acid.
 19. A method according to claim 16 wherein the ionizable atomic group is a basic group.
 20. A method for the preparation of an absorbing substance from lignocellulosic material, comprising the steps of: (a) treating said lignocellulosic material with alkali to remove at least a part of its lignin, hemicellulose, or a combination thereof; (b) after step (a), treating said lignocellulosic material with an oxidative chemical to provide its cellulose content with reactive radicals capable of functioning as polymerization initiators; (c) after step (b), adding to said lignocellulosic material at least one polymerizable monomer and at least one cross-linking agent to form a composition; and (d) polymerizing said composition
 21. A method for the preparation of an absorbing substance from lignocellulosic material, comprising the steps of: (a) treating said lignocellulosic material with alkali to remove at least a part of its lignin, hemicellulose, or a combination thereof; (b) after step (a), treating said lignocellulosic material to provide its cellulose content with reactive radicals capable of functioning as polymerization initiators; (c) after step (b), adding to said lignocellulosic material at least one polymerizable monomer and at least one cross-linking agent to form a composition, wherein said polymerizable monomer comprising an ionizable atomic group; and (d) polymerizing said composition. 